1. Field of the Invention
The present invention relates to substrates for cutting elements for use in earth-boring drill bits, to cutting elements incorporating such substrates, and to drill bits so equipped. Particularly, the present invention relates to erosion-resistant and abrasion-resistant cutting element substrates that can also absorb impacts that may be incurred during drilling. More particularly, the present invention relates to cutting elements including substrates having an erosion-resistant and abrasion-resistant region adjacent a periphery of an associated superabrasive cutting table and a more ductile, impact absorbent internal region.
2. Background of Related Art
Cutting element substrates have conventionally been employed as a means for securing cutting elements upon the faces of rotary drag bits in appropriate locations and orientations. Conventional cutting element substrates typically include a cemented carbide base and a superabrasive cutting table of desired configuration secured to the base in a desired orientation. Typically, the cemented carbide bases of these cutting element substrates include a single type of cemented carbide. When the base of a conventional cutting element substrate includes a tungsten carbide matrix and a cobalt binder, the erosion-resistance; abrasion-resistance, and toughness or ductility of the cutting element depend upon the grain size of the tungsten carbide and the relative proportions of tungsten carbide and cobalt. As is well known in the art, cemented carbide structures that include carbides having smaller grain sizes are more erosion-resistant and abrasion-resistant than cemented carbide structures with larger grains of carbide. Moreover, cemented carbides that include higher proportions of a binder, such as cobalt, are tougher, more ductile, and more impact resistant, and less erosion-resistant and abrasion-resistant than cemented carbides that include lesser amounts of binder.
Accordingly, conventional cutting element substrates, which typically include only one type of cemented carbide, may lack either ductility, toughness, and impact resistance or erosion-resistance and abrasion-resistance. Cutting element substrates with good erosion-resistance and abrasion-resistance but relatively low ductility resist excessive wearing behind the superabrasive cutting tables secured thereto and, therefore, prevent breaking away of the superabrasive cutting tables supported thereby, thus imparting the superabrasive cutting tables with an increased cutting life. Erosion-resistant and abrasion-resistant cutting element substrates are, however, relatively more prone to fracturing or shattering under impacts that may be incurred during drilling than more ductile cutting element substrates. More specifically, when erosion-resistant and abrasion-resistant materials, which typically have low impact resistance, are employed as the cutting element substrate, if a superabrasive cutting table is exposed to a fracture-generating impact, the adjacent substrate may also likely fracture. Thus, the superabrasive material of the cutting tables that are secured to the erosion-resistant and abrasion-resistant cutting element substrates, as well as the substrates themselves, may be damaged or lost prior to the ends of the useful lives of the cutting tables or of the entire cutting elements.
While more ductile cutting element substrates may better withstand the impacts that may be incurred during drilling, the relatively low erosion-resistance and abrasion-resistance of more ductile cutting element substrates may cause them to wear undesirably fast, especially at the exposed peripheral regions thereof, located adjacent the superabrasive cutting tables disposed thereon. Thus, the superabrasive cutting tables that are secured to these more ductile cutting element substrates may become unsupported proximate their locations of contact with a formation being drilled and may, therefore, be broken during cutting. Consequently, the useful lives of superabrasive cutting tables that are disposed on more ductile cutting element substrates may be reduced.
U.S. Pat. No. 4,359,335 (hereinafter "the '335 Patent"), which issued to Lloyd L. Garner on Nov. 16, 1982, discloses a rock bit insert that includes a cemented carbide base of a first composition with a more erosion-resistant and abrasion-resistant wear pad secured thereto and acting as a cutting surface. Thus, the wear pad of the insert of the '335 Patent is erosion-resistant and abrasion-resistant and is, therefore, useful for contacting a formation material in a borehole and for preventing boring of an undersized borehole. The wear pad does not, however, comprise a superabrasive material. Nor is the structure of the '335 Patent suitable for use as a cutting element for rotary drag bits that may be employed to bore very hard, abrasive formations.
U.S. Pat. No. 5,431,239 (hereinafter "the '239 Patent"), which issued to Gordon A. Tibbitts et al. on Jul. 11, 1995, discloses a cutting element substrate that includes an inner core of a material of enhanced fracture toughness surrounded by an outer layer of abrasion resistant material. The inner core of the substrate extends substantially through the length thereof. The materials of the inner core and the outer layer of the substrate have different coefficients of thermal expansion. According to the '239 Patent, upon cooling the materials from an elevated temperature, the material of the outer layer contracts or shrinks more quickly than the material of the inner core. Thus, an interference fit secures the outer layer to the inner core. However, undesirable residual stresses exist in the substrate of the cutting element of the '239 Patent due to the use of materials having different coefficients of thermal expansion.
Similar cutting element substrates are disclosed in U.S. Pat. No. 5,492,188 (hereinafter "the '188 Patent"), which issued to Redd H. Smith on Feb. 20, 1996. One embodiment of cutting element substrate disclosed in the '188 Patent includes three concentrically alignable structures. The center structure is configured as a ring and is fabricated from a tough and ductile material such as a metal or metal substrate. The innermost of the three structures is fabricated from an erosion-resistant and abrasion-resistant material such as a cemented carbide and extends substantially through the length of the substrate. In fabricating this embodiment of the substrate, the three structures are independently fabricated, then aligned and assembled with one another. These structures may then be secured to one another by means of high pressure and high temperature processes. An assembly method requiring alignment of the three structures is somewhat undesirable, however, in that additional fabrication time is required and the cutting elements cannot, therefore, be fabricated very efficiently.
On information and belief, there exists another cutting element including a substrate having a tough, ductile, and impact resistant outer shell and an erosion-resistant and abrasion-resistant inner core. This configuration is, however, somewhat undesirable since the outer shell may wear during use and, therefore, fail to adequately support the cutting table disposed thereon, while the inner core provides little useful impact resistance.
Accordingly, there is a need for a cutting element substrate with an erosion-resistant and abrasion-resistant peripheral region to provide better edge support for a superabrasive cutting table disposed on the cutting element and with a more ductile interior optimized to absorb residual stresses throughout the superabrasive cutting table and, thereby, maximize the useful life of the cutting element substrate and the superabrasive cutting table disposed thereon. There is also a need for such a cutting element substrate that can be efficiently fabricated. There are further needs for cutting element substrates that better withstand temperature changes and for cutting elements that are easily brazeable to a bit body.